This invention refers to an oil separating device manufactured from a corrosion-resistant and temperature-resistant material, which is used to separate oil and fat, respectively, from a liquid mixture, having a pre-separation chamber in which terminates an inlet for the oily liquid mixture to be separated and in which floating oil and fat, respectively, is separated from the feed liquid mixture, a microfiltration stage which separates emulsified oil and fat droplets, respectively, from the liquid mixture with the aid of a filter membrane that has a pore size of 0.1-2 .mu.m, preferably approximately 0.8 .mu.m, and a permeate discharge to carry away the liquid mixture, which has been purified from the oil and grease, respectively, out of the oil separating device.
Such a device, is for instance, known from the data sheet "Ultrafiltration and Microfiltration" of the Gutling company published in November 1991. The ultrafiltration and microfiltration methodology is also known from the German publication "Abtrennung von Feststoffen aus dem Abwasser [Separation of Solids from Effluent] 60. Siedlungswasserwirtschaftliches Kolloquium, Forschungs- und Entwicklungsinstitut fur Industrieund Siedlungswirtschaft sowie Abfallwirtschaft e.V. Stuttgart", R. Oldenburg Verlag, Munchen 1985, pages 294-304.
The known device serves to separate oils, fats and solids from emulsions, especially oil/water mixtures. Such oily liquid mixtures are widely used as cooling lubricants in the metal fabricating industry. After mechanical shaping and finishing, it is necessary to clean the fabricated components. In the past, this was usually carried out in cleaning equipment which utilized chlorinated solvents as the cleaning medium. Due to environmental concerns, these chlorinated hydrocarbon based cleaners are increasingly being replaced by aqueous alkali cleaners. However, these cleaners have only a limited absorption capacity for oil, it is necessary to dump and treat them before disposal. If, however, such a cleaning solution is pumped continuously at a relatively high velocity and at a controlled pressure across a porous membrane, it is possible, if the pore size is suitable, to retain the oil while the cleaning solution is being recycled into the process. Ultrafiltration employs pore sizes of 0.03-0.05 .mu.m, which also retains many of the surfactants. This effect is desirable where regulatory discharge standards have to be met, but it interferes with the recycling process. If, however, a relatively open microfiltration membrane with a pore size of between 0.1 and 2 .mu.m is selected, then the surfactants can pass completely, which is beneficial for recycling purposes, even if accompanied by a certain amount of oil slippage. In this way, it is theoretically possible to achieve an "infinite" service life of the cleaning bath when using microfiltration in conjunction with replenishment of the bath constituents. In practice, a factor of .times.5 to .times.10 in the service life can be achieved, which is a significant contribution to attaining an environmentally friendly cleaning process.
In order to reduce the loading of the microfiltration stage, the known oil separating device is provided with a pre-separation stage ahead of the microfiltration stage in which the feed liquid mixture is retained for a relatively long time (circa 1/4 to 1/2 hour), so that the non-emulsified oil can float to the top, and it can then be removed. For this reason only the emulsified oil has to be separated from the liquid mixture in the microfiltration state.
Since the liquid mixtures which are to be treated are normally either acidic or strongly alkaline and almost always very corrosive, the traditional oil separating devices are usually made out of plastic. This has, however, the disadvantage that hot liquid mixtures with temperatures above 60.degree. C., as they often occur in industrial applications, must then be cooled down before entering the oil separating device. A further disadvantage with the traditional devices is that they require a relatively large space for the pre-separation stage and the subsequent microfiltration stage. The traditional oil separating devices are therefore mainly suited for treating large volumes of liquids by way of a central treatment operation, which, however, makes them unsuitable for increasing the service life of smaller liquid volumes because of the larger space requirement, and they are therefore uneconomical and too expensive.
In addition, there is a relatively large maintenance requirement, due to the regular cleaning and replacement of the polymeric membranes that are normally used in the microfiltration stage, where the oil clings to the surfaces from where it must typically be removed every 100 operating hours.
Another maintenance expense with the traditional oil separating devices is the need for the relatively frequent replacement of the floating ring seals in the externally located pumps which are normally used to transport the liquid to be cleaned and to generate the pressure for introducing the liquid mixture into the microfiltration stage. The surfactants which are normally added to the cleaning solutions often crystallize on the seals causing grooves in the gasket rings during operation of the pump and subsequent leakage of the latter. The otherwise conceivable reason to use magnetically coupled pumps instead of pumps with floating ring seals also does not offer an answer to the problem, since the magnetic swarf which is typically present in the liquids to be treated can lead to damage of the pump heads.